In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an imagewise way on a receiving substrate, the latter not bearing any imagewise latent electrostatic image. In the case that the substrate is an intermediate endless flexible belt (e.g. aluminium, polyimide etc.), the imagewise deposited toner must be transferred onto another final substrate. If, however, the toner is deposited directly on the final receiving substrate, a possibility is fulfilled to create directly the image on the final receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
A DEP device is disclosed in e.g. U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called insulation layer; PA0 a shield electrode consisting of a continuous layer of conductive material on one side of the insulation layer; PA0 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the insulation layer; and PA0 at least one row of apertures. PA0 i) said printing apertures have a longest dimension A, measured on said side of said insulating material carrying said shield electrode and have a longest dimension D, measured on said side of said insulating material carrying said control electrodes, PA0 ii) said shield electrode has openings with a dimension B, measured parallel to said longest dimension A, said dimension B being equal to or larger than said dimension A, PA0 iii) said control electrodes have openings with a dimension E measured parallel to said longest dimension D, said dimension E being equal to or larger than said dimension D, PA0 iv) in each of said openings at least one printing aperture is present, and PA0 v) for each of said printing apertures present in each of said openings, B/A.gtoreq.1.10 and E=D.
Each control electrode is formed around one aperture and is isolated from each other control electrode.
Selected potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrode may face the receiving member substrate. A DC field is applied between the printhead structure and a single back electrode on the receiving member support. This propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the back electrode. The printhead structure as described in U.S. Pat. No. 3,689,935 is thus characterised by the presence of two electrode layers and is called hereinafter a P2-printhead structure. The voltages used for image-wise deposition of toner particles are of the order of about 400 V. Such devices have e.g. been described in U.S. Pat. No. 4,755,837.
DEP devices according to the principle, disclosed in U.S. Pat. No. 3,689,935, but using only a single electrode layer, with only control electrodes and no shield electrode have also been described. In e.g. U.S. Pat. Nos. 5,099,271, 5,402,158, EP-A 587 366 and EP-A 617335, devices have been described that operate according to the DEP principle with typical voltages of the order of 50 to 100 V. These printhead structures made from polyimide foils with apertures and control electrodes in a single plane are called further on P1-printhead structures. P1 printhead structures are characterised by a lower voltage needed to get toner images on the final receiver, but also by a higher contrast: i.e. the number of shades of grey between maximum density and minimum is rather low, typically binary.
A DEP device according to the P2-design is well suited to print half-tone images. The density variations present in a half-tone image can be obtained by modulation of the voltage applied to the individual control electrodes. Providing printing apertures in a DEP printhead structure comprising two electrodes (control electrode and shield electrode) separated by an insulating plastic material, to yield a printhead capable of producing images with high resolution and also with uniform density pattern is not an obvious process.
All printing apertures in the printhead structure must have exactly the predetermined diameter, the electrodes must stay in place and have a well defined and constant shape, and the walls of the printing apertures through the insulating plastic must be smooth to avoid clogging of the printing apertures. After forming the printing apertures in the printhead structure, each aperture must be individually addressable such as to be able to yield any density between zero and maximum density. Moreover every printing aperture has to be addressable to the same extent in order to yield smooth density pattern. Applying a controlling voltage of a few hundred of volts between an individual control electrode and the global shield electrode may not short-circuit the nozzle and render it useless.
Printhead structures made from flexprint material, but with a much more complicated design have also been described in the literature. In U.S. Pat. No. 4,912,489 e.g. a printhead structure of polyimide with 3 electrode layers is described. A first sheet of polyimide has a printing aperture having on one side a common shield electrode, and on the other side individual control electrodes. A second sheet of polyimide is laminated upon said first sheet of flexprint material and has printing apertures with the same aperture diameter and registered with a high degree of accuracy with said first sheet with printing apertures. At the side facing away from said first sheet of flexprint material screening electrodes are available, said screening electrodes having a diameter that is larger than the diameter of said apertures.
In U.S. Pat. No. 5,170,185 a printhead structure is described consisting of two sheets of polyimide foil laminated to each other. Both sheets have printing apertures with the same aperture diameter, and both of said printing apertures have to be registered to a high degree of accuracy. A common shield electrode is provided at a first side of said first flexprint material facing away from said lamination side. Said second sheet of flexprint material has individual control electrode at the other side of said laminated printhead structure, also facing away from said lamination side. Said control electrodes in said second sheet of flexprint material have conductive patterns inside said printhead structures as depicted in FIG. 23 said U.S. Pat. No. 5,170,185.
In U.S. Pat. No. 5,038,159 a printhead structure is made from a single sheet of flexprint material but the shape of said printing apertures is made concave in one embodiment of this invention. The aperture diameter is larger at the side of the common shield electrode than at the side of the individual control electrodes. The printing aperture is made in said plastic material in such a way that a concave form is obtained. In a second embodiment of said invention a single sheet of flexprint material is used. The printing aperture has a fixed diameter and the individual control electrodes are through-hole-connected to the shield electrode side. Said shield electrode itself has a much larger diameter so that it remains electrically insulated from said control electrode. This printhead structure is also illustrated in FIG. 2 of said U.S. Pat. No. 5,038,159.
There is thus still a need for a DEP system, using a printhead structure comprising two electrodes (control electrode and shield electrode) separated by an insulating plastic material and wherein printing apertures are present, wherein the printing apertures are not easily clogged by the toner particles and wherein each aperture is individually addressable in a reproducible way by low control voltages, and wherein an image with enhanced grey scale resolution can be obtained, and wherein said printhead structure can be fabricated in an easy and straightforward way.